Display-integrated type tablet device

ABSTRACT

A detection and hold circuit outputs a reference signal having a level proportional to an x-coordinate detection pulse input from an X-coordinate signal amplifier. A delay element delays the reference signal. An adder adds the x-coordinate detection pulse from the X-coordinate signal amplifier and a delayed signal from the delay element. Comparators obtain a coordinate detection pulse making a detection signal from another detection and hold circuit for an LCD voltage inversion pulse serve as a reference signal. Thus noise following the x-coordinate detection pulse is removed to generate a coordinate detection pulse with a reference value corresponding to the level of the LCD voltage inversion pulse to allow a stable coordinate detection to be achieved. A correction signal generation circuit reads out correction data stored in an internal memory and transmits the data as a correction signal to a correction voltage generation circuit in an x-coordinate detection period when the segment electrode placed in the lower position is scanned. The correction voltage generation circuit generates a correction voltage for canceling a voltage induced at the common electrode placed in the upper position based on the input correction signal and a bias power source input from a DC power supply circuit.

This is a divisional of application Ser. No. 08/065,610 filed May 21,1993 now U.S. Pat. No. 5,410,329.

BACKGROUND OF THE INVENTION

The present invention relates to a display-integrated type tablet devicefor use in a personal computer, a word processor, or the like.

As a means for inputting a handwritten letter or a figure into acomputer or a word processor, there has been put into practical use adisplay-integrated type tablet device which is formed by laminating anelectrostatic induction type tablet on a liquid crystal display and iscapable of receiving an input of a letter or a figure into itselectrostatic induction type tablet as if the letter or figure werewritten on paper by writing implements. However, in such adisplay-integrated type tablet device, electrodes are viewed as agrating on the display screen due to a difference in reflectance ortransmittance between a portion having an electrode and a portion havingno electrode, which has been a cause of degrading the quality of animage displayed on the liquid crystal display screen.

As a tablet free of the above-mentioned drawback, lately adisplay-integrated type tablet device as shown in FIG. 15 is proposed bythe applicant of the present invention (Japanese Patent Application No.3-46751 and a co-pending U.S. patent application Ser. No. 07/849,733)though it is not yet well known. It should be noted that theabove-mentioned device is not a prior art, and herein described forbetter understanding of the present invention.

In the above-mentioned display-integrated type tablet device, electrodesconcurrently serve as image display electrodes of a liquid crystaldisplay (LCD) and as coordinate detection electrodes of an electrostatictype tablet device. There are provided in one frame period a coordinatedetection period when designated coordinates on the tablet are detectedand an image display period when an image is displayed as shown in FIG.16 to time-sharingly effect the coordinate detection and image display.

Referring to FIG. 15, an LCD panel 101 is constructed by interposingliquid crystals between common electrodes Y₁ through Y_(n) (an arbitrarycommon electrode represented by Y hereinafter) and segment electrodes X₁through X_(m) (an arbitrary segment electrode represented by Xhereinafter) which are arranged at right angles to each other, in whicheach portion where a common electrode Y and a segment electrode X crosseach other constitutes each pixel. In other words, n×m dot pixels arearranged in matrix in the LCD panel 101.

The above-mentioned display-integrated type tablet device has anadvantage of permitting cost reduction as well as compact and lightweight design by virtue of the concurrent use of the electrodes anddrive circuits as those of the liquid crystal display and those of theelectrostatic induction type tablet in addition to an advantage ofmaking the grating-shaped electrodes invisible for a better imagepresentation in contrast to the conventional type formed by laminatingthe electrostatic induction type tablet on the liquid crystal display.

The above-mentioned display-integrated type tablet device operates asfollows. A common electrode drive circuit 102 for driving the commonelectrode Y and a segment electrode drive circuit 103 for driving thesegment electrode X are connected to a display control circuit 105 and adetection control circuit 106 via a switching circuit 104. The switchingcircuit 104 is controlled by a control circuit 107 so that it outputs anoutput signal from the display control circuit 105 to the commonelectrode drive circuit 102 and the segment electrode drive circuit 103in an image display period or outputs an output from the detectioncontrol circuit 106 to the common electrode drive circuit 102 and thesegment electrode drive circuit 103 in a coordinate detection period.

Although the switching circuit 104, the display control circuit 105, thedetection control circuit 106, and the control circuit 107 are expresseddividedly in blocks in FIG. 15, the circuits are integrated in an LSI(Large Scale Integrated) circuit in practice. Therefore, the LSI cannotbe strictly sectioned into such blocks in a practical circuitarrangement.

In the aforementioned image display period, a mode signal (mode) outputfrom the control circuit 107 to the segment electrode drive circuit 103and to the switching circuit 104 is switched to an image display mode.Consequently, the segment electrode drive circuit 103 selects the imagedisplay mode, while the switching circuit 104 switches so as to selectand output an output signal from the display control circuit 105.

Then there are output, from the display control circuit 105, shift datas from a shift data output terminal S, an inversion signal fr from aninversion signal output terminal FR, a clock signal cp1 from a clockoutput terminal CP1, a clock signal cp2 from a clock output terminalCP2, and display data D₀ through D₃ from data output terminals D0through D3.

The above-mentioned clock signal cp1 is a clock signal which has aperiod when pixels in one line display an image, and the signal is inputas a clock signal cp1o to a clock input terminal YCK of the commonelectrode drive circuit 102 and a latch pulse input terminal XLP of thesegment electrode drive circuit 103 via an output terminal CP1O of theswitching circuit 104. The shift data s which is a pulse signal forselecting a specified common electrode Y is input as shift data (so) toa shift data input terminal DIO1 of the common electrode drive circuit102 in synchronization with the clock signal cp1o via an output terminalSO of the switching circuit 104.

When the shift data so is input to the common electrode drive circuit102, the pulse position of the shift data so is shifted in a shiftregister in synchronization with the clock signal cp1o, and drive pulsesof a common electrode drive signal are applied to the common electrodesY₁ through Y_(n) from output terminals O1 through On of the commonelectrode drive circuit 102 in correspondence with the shift position.The common electrode drive signal is generated based on bias powersources V₀ through V₅ supplied from a DC power supply circuit 112.

The above-mentioned clock signal cp2 is a clock signal which has aperiod being a division of a period when pixels in one line displays animage, and the signal is input as a clock signal cp2o to a clock inputterminal XCK of the segment electrode drive circuit 103 via an outputterminal CP2O of the switching circuit 104.

The image display data D₀ through D₃ are input as display data D₀ othrough D₃ o to input terminals XD0 through XD3 of the segment electrodedrive circuit 103 via output terminals D0O through D3O of the switchingcircuit 104, and then successively taken into a register in the segmentelectrode drive circuit 103 in synchronization with the clock signalcp2o. When all the image display data corresponding to the pixels in oneline are taken in, the display data taken in are latched at a timing ofthe clock signal cp1o input to the latch pulse input terminal XLP. Thendrive pulses of the segment electrode drive signal corresponding to thedisplay data are applied from output terminals O1 through Om of thesegment electrode drive circuit 103 to the segment electrodes X₁ throughX₂. The segment drive signal is also generated based on the bias powersources V₀ through V₅ supplied from the DC power supply circuit 112.

It is noted that the inversion signal fr is a signal for preventing thepossible deterioration of the liquid crystals due to electrolysis byperiodically inverting the direction of voltage application to theliquid crystals in the image display period. The inversion signal fr isinput as an inversion signal fro to an inversion signal input terminalYFR of the common electrode drive circuit 102 and an inversion signalinput terminal XFR of the segment electrode drive circuit 103 via aninversion signal output terminal FRO of the switching circuit 104.

Thus the pixel matrix of the LCD panel 101 is line-sequentially drivenby the operations of the above-mentioned common electrode drive circuit102 and the segment electrode drive circuit 103 to display an imagecorresponding to the display data D₀ through D₃ on the LCD panel 101.

In the aforementioned coordinate detection period, the mode signal(mode) output from a control circuit 107 to the segment electrode drivecircuit 103 and to the switching circuit 104 is switched to a coordinatedetection mode. Consequently, the segment electrode drive circuit 103selects the coordinate detection mode, while the switching circuit 104switches so as to select and output an output signal from the detectioncontrol circuit 106.

Then there are output, from the detection control circuit 106, shiftdata sd from a shift data output terminal Sd, an inversion signal frdfrom an inversion signal output terminal FRd, a clock signal cp1d from aclock output terminal CP1d, a clock signal cp2d from a clock outputterminal CP2d, and drive data D₀ d through D₃ d from data outputterminals D0d through D3d.

The clock signal cp1d is a clock signal which has a period when onecommon electrode Y or one segment electrode X is scanned, and the signalis input as the clock signal cp1o to the clock input terminal YCK of thecommon electrode drive circuit 102 and the latch pulse input terminalXLP of the segment electrode drive circuit 103 via the output terminalCP1O of the switching circuit 104. Meanwhile, the shift data sd which isa pulse signal for selecting a specified common electrode Y or segmentelectrode X is input as the shift data (so) to the shift data inputterminal DIO1 of the common electrode drive circuit 102 or to a datainput terminal EIO1 of the segment electrode drive circuit 103 via theoutput terminal SO of the switching circuit 104 in synchronization withthe aforementioned clock signal cp1d.

Then, in the same manner as described above, the pulse position of theshift data so input to the common electrode drive circuit 102 is shiftedin a shift register in synchronization with the clock signal cp1o, andscanning pulses of common electrode drive signals y₁ through y_(n)(arbitrary common electrode scanning signal represented by yhereinafter) are successively applied from the output terminals O1through On corresponding to the shift position to the common electrodesY₁ through Y_(n). The common electrode scanning signal y is generatedbased on the bias power sources V₀ through V₅ supplied from the DC powersupply circuit 112.

Meanwhile, the pulse position of the shift data so input to the segmentelectrode drive circuit 103 which has been selected to be in thecoordinate detection mode is shifted in a shift register insynchronization with the clock signal cp1o, and scanning pulses ofsegment electrode drive signals x₁ through x_(n) (arbitrary segmentelectrode scanning signal represented by x hereinafter) are successivelyapplied from the output terminals O1 through Om corresponding to theshift position to the segment electrodes X₁ through X_(m).

Although the above described the case where the segment electrode X isscanned based on the shift data so and the clock signal cp1o, thesegment electrode X may be scanned in the following manner. That is, thesegment electrode scanning signal x is output to the segment electrodesX₁ through X_(m) from the output terminals O1 through Om of the segmentelectrode drive circuit 103 while making any bit of the drive data D₀ dthrough D₃ d output from the detection control circuit 106 serve as theshift data sd and making the clock signal cp2d serve as a sync signal.

In the above case, the clock signal cp2d is a clock signal which has aperiod when the segment electrode X is scanned, and the signal is inputas the clock signal cp2o to the clock input terminal XCK of the segmentelectrode drive circuit 103 via the output terminal CP2O of theswitching circuit 104.

The drive data D₀ d through D₃ d are input as drive data D₀ o through D₃o to the input terminals XD0 through XD3 of the segment electrode drivecircuit 103 via the output terminals D0O through D3O of the switchingcircuit 104, and then successively taken into the register of thesegment electrode drive circuit 103 in synchronization with the clocksignal cp2o. Then scanning pulses of the segment electrode scanningsignals x₁ through x_(m) corresponding to the above-mentioned drive dataare output from the output terminals O1 through O_(m) of the segmentelectrode drive circuit 103 to segment electrodes X₁ through X_(m). Thesegment electrode scanning signal x is also generated based on the biaspower sources V₀ through V₅ supplied from the DC power supply circuit112.

It is noted that the output terminal EIO2 of the segment electrode drivecircuit 103 is the output terminal of the final stage of the shiftregister, and the output terminal EIO2 outputs a pulse signal seio2having the same pulse width as that of the shift data sd as shown inFIG. 18.

The segment electrode scanning signal x is also generated based on thebias power sources V₀ through V₅ supplied from the DC power supplycircuit 112.

FIG. 20 is a timing chart of the scanning signals in the coordinatedetection period of the above-mentioned display-integrated type tabletdevice. The coordinate detection period is separated into anx-coordinate detection period and a subsequent y-coordinate detectionperiod. In the x-coordinate detection period, scanning pulses of thesegment electrode scanning signal x are successively applied to thesegment electrode X. In the y-coordinate detection period, scanningpulses of the common electrode scanning signal y are successivelyapplied to the common electrode Y.

In the above case, the scanning pulse voltage of the segment electrodescanning signal x or the common electrode scanning signal y for one ofthe segment electrode X or the common electrode Y to be scanned (thevoltage referred to as the "scanning voltage" hereinafter) is set at thebias power source V₅ supplied from the DC power supply circuit 112. Onthe other hand, the scanning pulse voltage of the segment electrodescanning signal x or the common electrode scanning signal y for theother of the segment electrode X or the common electrode Y to be notscanned (the voltage referred to as the "non-scanning voltage"hereinafter) is set at the bias power source V₁ supplied from the DCpower supply circuit 112.

With application of the above-mentioned scanning signal V₅, a voltage isinduced at a designation coordinate detection pen (referred to merely asthe "detection pen" hereinafter) 108 as shown in FIG. 17(b) due to afloating capacitance between the segment electrode X or the commonelectrode Y and a tip electrode of the detection pen 108 as shown inFIG. 17(a). The voltage induced at the detection pen 108 is amplified inan amplifier 109 and then converted into a binary signal as shown inFIG. 17(c) to be input to an x-coordinate detection circuit 110 and ay-coordinate detection circuit 111.

The x-coordinate detection circuit 110 and the y-coordinate detectioncircuit 111 detect the x-coordinate value or the y-coordinate value of aposition designated by the detection pen 108 by detecting a period "T"from the time when the scanning voltage signal V₅ is applied to the timewhen an induction voltage takes its maximum value based on an outputfrom the amplifier 109 and a timing signal from the control circuit 107.

FIG. 21 shows a relation in position between the LCD panel 101 and thedetection pen 108. When directly touching the LCD panel 101 with thedetection pen 108, a stress is applied to a polarizer 114 and an LCDenclosure glass plate 115 to modulate the transmittance of the LCD panel101, and consequently so-called a Newton-ring-shaped pattern appears.Therefore, a transparent protection plate 116 such as glass or acrylicresin is interposed between the LCD panel 101 and the detection pen 108to provide an air gap 117 so as not to apply any stress onto the LCDpanel 101.

However, in the display-integrated tablet construction as shown in FIG.21, the stress onto the LCD panel 101 can be avoided but the transparentprotection plate 116 is significantly deformed by the stress, whichresults in greatly varying the distance between the segment electrode Xas well as the common electrode Y (referred to merely as the "scanningelectrode" hereinafter) and a detection electrode 113 of the detectionpen 108. In such a display-integrated type tablet device, coordinatedetection is effected taking advantage of an electrostatic inductionphenomenon. Therefore, the output of the detection pen 108 varies inreverse proportion to the distance between the scanning electrodes X andY and the detection electrode 113.

Therefore, in a normal system where the analog output of the detectionpen 108 is converted into a binary signal with a fixed threshold toobtain a detection pulse, there is a problem that a secured detectionpulse cannot be obtained for a detection signal having a lowsignal-to-noise ratio.

In the above case, there can be presented an electrical equivalentcircuit of the LCD panel 101 in the x-coordinate detection period whenthe common electrodes is fixed at the voltage V₀ and the segmentelectrodes receive a voltage which changes from the voltage V₀ to thevoltage V₅ and again to the voltage V₀ as shown in FIG. 22. It is notedthat rcd₁ through rcd_(n) are internal resistance values of the commonelectrode drive circuit 102, rsd₁ through rsd=are internal resistancevalues of the segment electrode drive circuit 103, rc_(1'1) throughrc_(n'm) are resistance values of the common electrodes, rs_(1'1)through rs_(n'm) are resistance values of the segment electrodes, andeach of C_(1'1) through C_(n'm) is a capacitance of one pixel of theLCD.

In the above case, by ignoring the resistance values rs_(1'1) throughrs_(n'm) and assuming rc_(1'1) =rc_(1'2) = . . . =rc_(n'm) =rc, rcd₁=rcd₂ = . . . rcd_(n) =rcd, C_(1'1) =C_(1'2) = . . . =C_(n'm) =C, andrsd₁ =rsd₂ = . . . =rsd_(m) =rsd, the equivalent circuit as shown inFIG. 22 can be further simplified as shown in FIG. 23.

Referring to FIG. 23, each of the capacitors of segment electrodes towhich is applied the voltage V₀ has no electric charge, while each ofthe capacitors of the segment electrodes to which is applied the voltageV₅ has an electric charge of (V₅ -V₀)/n/C. It is noted that the voltageV₅ is simultaneously applied to four segment electrodes in FIG. 23.

When the scanning of the segment electrodes progresses by one clockpulse in the condition as shown in FIG. 23, a condition as shown in FIG.24 is achieved. Consequently, there is formed a discharge currentthrough the capacitor of the segment electrode X at which the electricpotential has changed from the voltage V₅ to the voltage V₀. Meanwhile,there is formed a charge current through the capacitor of the segmentelectrode X at which the electric potential has changed from the voltageV₀ to the voltage V₅. Since rc/n can be ignored in comparison to rsd,the above-mentioned charge and discharge currents do not flow into thecommon driver.

However, as shown in FIG. 25, the charge and discharge currents of thecapacitors flow into the common electrode drive circuit 102 at the timeof starting the scanning of the segment electrode X (the voltage V₅ isapplied only to the segment electrode X₁ closest to the common electrodedrive circuit 102), and therefore a voltage drop takes place because ofthe output impedance of the common electrode drive circuit 102. Such avoltage drop is superimposed on the entire common electrodes.

A voltage drop due to the resistance of the common electrodes isadditionally superimposed at the time of ending the scanning of thesegment electrode X (the voltage V₅ is applied only to the segmentelectrode X_(m) farthest from the common electrode drive circuit 102) asshown in FIG. 26, and therefore an increased amount of noise takes placeat the scanning end time as compared with the scanning start time.

In an arrangement of the LCD panel 101, the transparent protection plate116, and the detection pen 108 as shown in FIG. 21, the electrostaticcoupling between the detection electrode 113 of the detection pen 108and the common electrodes is strong, and the electrostatic couplingbetween the detection electrode 113 and the segment electrodes is veryweak. Therefore, the noise superimposed on the common electrodes exertsgreat influence. In the normal system where the analog output of thedetection pen 108 is converted into a binary signal with a fixedthreshold to obtain a coordinate pulse, the detection signal induced atthe detection pen 108 in scanning the segment electrodes has a degradedsignal-to-noise ratio as shown in FIG. 27.

The above fact also results in a problem that no secured coordinatepulse for detecting the x-coordinate value can be obtained.

In the above-mentioned electrostatic induction type tablet, thedetection signal of the detection pen 108 is obtained as a signal ofwhich DC component is lost. In the output of the detection pen in thedisplay-integrated type tablet device employing an electrostaticinduction type tablet, an LCD image display polarity inversion pulse, ay-coordinate detection pulse, and an x-coordinate detection pulse can beobtained as analog signals respectively in the image display period, they-coordinate detection period, and the x-coordinate detection period.Therefore, a delay of a low-frequency component takes place due to acapacitive coupling in the x-coordinate detection circuit 110 and they-coordinate detection circuit 111, and each of the pulses exerts alow-frequency interference with the other pulses, which results in theappearance of an error in coordinate detection.

FIG. 18 shows a timing chart of a segment electrode scanning signal xapplied to the segment electrode X of the LCD panel 101 and waveforms ofthe output signal from the detection pen 108 or the amplifier 109 in thex-coordinate detection period. FIG. 19 shows an equivalent circuit ofthe common electrode drive circuit 102 and the segment electrode drivecircuit 103 in the x-coordinate detection period.

Referring to FIG. 19, switch units S₁, S₂, . . . S_(m) in the segmentelectrode drive circuit 103 successively apply the scanning voltage V₅to each segment electrode X, while switch units S₁ ', S₂ ', . . . S_(m)' in the common electrode drive circuit 102 successively apply thescanning voltage V₅ to each common electrode Y. The switch units areeach composed of a CMOS (Complementary Metal Oxide Semiconductor)silicon gate circuit.

It is noted that resistors r_(c1), r_(c5), r_(s1), and r_(s5) areon-resistors.

The above-mentioned switch units S₁, S₂, . . . , S_(m-1), S_(m) in thesegment electrode drive circuit 103 are switched to the voltage V₅sequentially from S₁ to S_(m). Thus the scanning voltage V₅ of thesegment electrode scanning signal x is applied to the segment electrodeX sequentially from a segment electrode X₁ located at an end of the LCDpanel 101 to a segment electrode X_(m) located at the other end as shownin FIG. 18. Referring to FIG. 19, the switch units S₂, S₃, and S₄ areswitched to the voltage V₅ to apply the scanning voltage to the segmentelectrodes X₂, X₃, and X₄, while the other switch units S₁, S₅, . . .S_(m) are switched to the voltage V₁ to apply the non-scanning voltageto the segment electrodes X₁, X₅, . . . X_(m).

When the segment electrode X is scanned in the above-mentioned manner, avoltage as shown by a waveform in FIG. 17(b) is induced at the detectionelectrode of the detection pen 108. The induction voltage is amplifiedin the amplifier 109 and then converted into a binary signal as shown bya waveform in FIG. 17(c). Then a peak time "T" of the induction voltageis obtained from the rise-time "T" and the fall-time "T₂ " of theobtained binary signal to calculate the x-coordinate value designated bythe tip end of the detection pen 108.

The induction voltage actually obtained through the segment electrodescanning has a waveform as shown by a waveform (d) in FIG. 18, wherenoise components F and R are detected at the scanning start time and atthe scanning end time.

FIG. 18 shows a waveform (e) obtained by differentiating twice theinduction voltage signal (referred to as the "detection signal"hereinafter) as shown by a waveform (d) in FIG. 18 in the amplificationstage of the amplifier 109 to obtain a binary pulse as narrow aspossible.

The voltage induced at the detection pen 108 when the detection pen 108is placed on the LCD panel 101 ideally has a waveform as shown by awaveform (c) in FIG. 18. A detection signal having such an idealwaveform can be obtained when scanning the electrodes placed in theupper position closer to the detection pen 108 as in the case of thecommon electrode Y as shown in FIGS. 15, 17(a), and 19.

However, when scanning the electrodes placed in the lower positionfarther from the detection pen 108 as in the case of the segmentelectrode X, there is obtained a detection signal having the undesirableinduction noise peaks (referred to as the "induction noise peak"hereinafter) F and R induced due to a voltage induced at the electrode(common electrode Y) placed closer to the detection pen 108 in additionto the peak S (referred to as the "detection peak" hereinafter) formedbased on the electrode scanning signal as shown by a waveform (d) inFIG. 18.

Therefore, when scanning a portion near the segment electrode X₁ or aportion near the segment electrode X_(m) located at end portions of theLCD panel 101, the detection signal peak S appears at the time ofstarting or ending the scanning in the x-coordinate detection period.Therefore, the induction noise peak F or the induction noise peak R issuperimposed on the detection signal peak S to result in a problem thatthe accuracy in detecting the x-coordinate by means of the tip end ofthe detection pen is degraded.

The induction noise peaks F and R as described above appear as follows.

Since the resistance of the on-resistors r_(c1) and r_(c5) (=about 1 kΩ)and the electrode resistance r of the common electrode Y as shown inFIG. 19 are great, when the segment electrode X in the lower position isscanned while switching the switch units S₁, S₂, . . . S_(m) of thesegment electrode drive circuit 103 successively to the voltage V₅,e.g., when the segment electrodes X₂, X₃, and X₄ are scanned, a currenti caused by the scanning voltage applied to the segment electrodes X₂,X₃, and X₄ flows through the segment electrodes X₂, X₃, and X₄ by way ofa floating capacitance C (about 10 pF/mm²) between the segment electrodeX and the common electrode Y as shown by the arrow in FIG. 19 and thento the DC power supply circuit 112 by way of the on-resistor r_(s5)inside the segment electrode drive circuit 103.

Therefore, the voltage at the common electrode Y which should be thenon-scanning voltage V₁ shifts slightly to the scanning voltage V₅ (tothe lower voltage side) of the segment electrode X as shown by awaveform (a) in FIG. 18.

Therefore, when the detection pen 108 is put close to the LCD panel 101in the condition as shown in FIG. 19, there is induced a voltage asshown by the waveform (d) in FIG. 18 containing the wavy induction noisepeaks F and R as shown by the waveform (b) in FIG. 18 attributed to thevoltage induced at the common electrode Y as shown by the waveform (a)in FIG. 18 in addition to the detection signal peak S at the detectionelectrode of the detection pen 108. As a result, the output signal fromthe amplifier 109 is to have a waveform (e) as shown in FIG. 18.

In contrast to the fact that the common electrode Y placed in the upperposition directly faces the detection electrode of the detection pen108, the segment electrode X placed in the lower position faces thedetection electrode of the detection pen 108 by way of a gap attributedto the common electrodes Y. Therefore, the electrostatic coupling forcebetween the common electrode Y and the detection electrode of thedetection pen 108 is much greater than the electrostatic coupling forcebetween the segment electrode X and the detection electrode. Therefore,despite that the voltage at the common electrode Y shifted toward theside of the lower voltage is extremely low as shown by the waveform (a)in FIG. 18, the voltage induced at the detection electrode based on theslight voltage variance has the same level as that of the detectionsignal peak S.

When the detection pen 108 is located in the center position of the LCDpanel 101, the induction noise peaks F and R can be separable from thedetection signal peak S as shown by the waveform (d) in FIG. 18, andtherefore the induction noise peaks F and R exert less influence.

However, when the detection pen 108 is located near the segmentelectrode X₁ or the segment electrode X_(m), the detection signal peak Sand the induction noise peaks F and R are superimposed on each other,the detection signal peak S is to have a complicatedly distortedwaveform. Therefore, even when the detection signal is converted into abinary signal, the correct x-coordinate of the tip of the detection pen108 cannot be detected.

Particularly, the induction noise peak R has a polarity reverse to thatof the detection signal peak S. Therefore, when the detection pen 108 islocated near the segment electrode X_(m), the detection signal peak Sand the induction noise peak R having opposite polarities aresuperimposed on each other to lower the level of the detection signalpeak S to significantly reduce the coordinate detection accuracy. In anextreme case, the x-coordinate value cannot be detected.

SUMMARY OF THE INVENTION

The present invention has been developed with a view to substantiallysolving the above described disadvantages. Accordingly, a first objectof the present invention is to provide an improved display-integratedtype tablet device having a high coordinate detection accuracy, thetablet being capable of performing a stable coordinate detection evenwhen distances between a detection electrode of a detection pen and asegment electrode and a common electrode vary, eliminating noisecomponents generated in an output signal from the detection pen takingplace at the time of ending the scanning, and preventing the possibleinterference of other pulses with a coordinate detection pulse of theabove-mentioned output signal.

In order to achieve the aforementioned first object, there is provided adisplay-integrated type tablet device including a display panel whichhas a display material interposed between segment electrodes and commonelectrodes crossing each other at right angles and is driven by a dutyratio control type drive method, a detection pen having at a tip of thedetection pen an electrode electrostatically coupled with the segmentelectrodes and the common electrodes of the display panel, a segmentelectrode drive circuit for driving the segment electrodes, a commonelectrode drive circuit for driving the common electrodes, a displaycontrol circuit for displaying an image on the display panel bycontrolling the segment electrode drive circuit and the common electrodedrive circuit in a period of displaying the image, a detection controlcircuit for controlling the segment electrode drive circuit tosequentially scan the segment electrodes of the display panel byapplying a scanning voltage successively to the segment electrodes andcontrolling the common electrode drive circuit to sequentially scan thecommon electrodes by applying a scanning voltage successively to thecommon electrodes in a coordinate detection period composed of a firstscanning period and a second scanning period, an x-coordinate detectioncircuit for detecting an x-coordinate value designated on the displaypanel by the tip of the detection pen according to an output signalgenerating timing of the detection pen and a scanning timing of thesegment electrodes, and a y-coordinate detection circuit for detecting ay-coordinate value designated on the display panel by the tip of thedetection pen according to an output signal generating timing of thedetection pen and a scanning timing of the common electrodes, whereinthe detection control circuit being capable of controlling the segmentelectrode drive circuit and the common electrode drive circuit so thatone which is positioned closer to the detection pen out of the segmentelectrodes and the common electrodes is scanned in the first scanningperiod and the other electrodes are scanned in the second scanningperiod subsequent to the first scanning period, the display-integratedtype tablet device comprising a detection circuit which receives anoutput signal from the detection pen to detect and hold a voltagevariation quantity of one of the segment electrodes and the commonelectrodes and outputs a reference signal corresponding to the voltagevariation quantity of the one of the segment electrodes and the commonelectrodes in the first scanning period, and the coordinate detectioncircuit which is relevant to the other of the segment electrodes and thecommon electrodes, out of the x-coordinate detection circuit and they-coordinate detection circuit, operating to detect a coordinatedetection pulse in the output signal from the detection pen based on thereference signal from the detection circuit.

Also there is provided a display-integrated type tablet device includinga display panel which has a display material interposed between segmentelectrodes and common electrodes crossing each other at right angles andis driven by a duty ratio control type drive method, a detection penhaving at a tip of the detection pen an electrode electrostaticallycoupled with the segment electrodes and the common electrodes of thedisplay panel, a segment electrode drive circuit for driving the segmentelectrodes, a common electrode drive circuit for driving the commonelectrodes, a display control circuit for displaying an image on thedisplay panel by controlling the segment electrode drive circuit and thecommon electrode drive circuit in a period of displaying the image, adetection control circuit for controlling the segment electrode drivecircuit to sequentially scan the segment electrodes of the display panelby applying a scanning voltage successively to the segment electrodesand controlling the common electrode drive circuit to sequentially scanthe common electrodes by applying a scanning voltage successively to thecommon electrodes in a coordinate detection period, an x-coordinatedetection circuit for detecting an x-coordinate value designated on thedisplay panel by the tip of the detection pen according to an outputsignal generating timing of the detection pen and a scanning timing ofthe segment electrodes, and a y-coordinate detection circuit fordetecting a y-coordinate value designated on the display panel by thetip of the detection pen according to an output signal generating timingof the detection pen and a scanning timing of the common electrodes, thedisplay-integrated type tablet device comprising a detection circuit forreceiving an output signal from the detection pen to detect and hold avoltage variation quantity of the segment electrodes and the commonelectrodes and outputting a reference signal corresponding to thevoltage variations of the segment electrodes and the common electrodesin the image display period, and the x-coordinate detection circuit andthe y-coordinate detection circuit operating to detect a coordinatedetection pulse in the output signal from the detection pen based on thereference signal from the detection circuit.

It is preferable that the x-coordinate detection circuit or they-coordinate detection circuit includes a detection circuit fordetecting and holding a coordinate detection pulse in the output signalfrom the detection pen and outputting a detection signal, a delaycircuit for delaying the detection signal from the detection circuit fora specified period of time, and a synthesis circuit for synthesizing thedetection signal from the delay circuit and the output signal from thedetection pen thereby suppressing noise following a peak of thecoordinate detection pulse in the detection signal.

Further it is preferable that each of the x-coordinate detection circuitand the y-coordinate detection circuit includes three clamp circuitswhich respectively operate in synchronization with the image displayperiod, a segment electrode scanning period, or a common electrodescanning period to take in either one of different signals of: theoutput signal from the detection pen in the image display period; theoutput signal from the detection pen in the segment electrode scanningperiod; or the output signal from the detection pen in the commonelectrode scanning period, thereby preventing mutual interference, whichpossibly occurs in the x-coordinate detection circuit or they-coordinate detection circuit, between a display voltage inversionpulse, an x-coordinate detection pulse, and a y-coordinate detectionpulse, which are forming a time series as included in the output signalfrom the detection pen.

According to the above-mentioned arrangement, the electrodes positionedon the side of the detection pen out of the segment electrodes or thecommon electrodes are scanned under the control of the detection controlcircuit in a first scanning period of the coordinate detection period.Then in the above time the output signal from the detection pen is inputto the detection circuit to detect and hold the voltage variationquantity of one electrode group and output a reference signal accordingto the voltage variation quantity of the one electrode group. Then thecoordinate detection pulse in the output signal from the detection penis detected based on the reference signal from the detection circuit bymeans of a coordinate detection circuit relevant to the other electrodegroup out of the x-coordinate detection circuit and the y-coordinatedetection circuit.

In the above-mentioned manner, based on the reference signal accordingto the voltage variation quantity of one electrode group at the time ofscanning the one electrode group, the coordinate detection pulsecontained in the output signal from the detection pen is detected by thecoordinate detection circuit relevant to the other electrode group.

Therefore, the variation in level of the output signal from thedetection pen due to the change of distance between the detection penand the segment electrode or the common electrode can be corrected bythe aforementioned reference signal.

Furthermore, according to the above-mentioned arrangement, the outputsignal from the detection pen in the image display period is input tothe detection circuit to output a reference signal according to thevoltage variation quantities of the segment electrode and the commonelectrode. Then based on the reference signal according to the voltagevariation quantities of the segment electrode and the common electrode,the coordinate detection pulse contained in the output signal from thedetection pen in the coordinate detection period is detected by thex-coordinate detection circuit and the y-coordinate detection circuit.

Furthermore, according to the above-mentioned arrangement, by means ofthe detection circuit of the x-coordinate detection circuit or they-coordinate detection circuit, the coordinate detection pulse containedin the output signal from the detection pen is detected and held. Thenthe detection signal from the detection circuit is delayed for aspecified period of time by the delay circuit. Thereafter, the detectionsignal from the delay circuit and the output signal from the detectionpen are synthesized by the synthesis circuit.

Thus the noise following the peak of the coordinate detection pulsecontained in the above-mentioned detection signal is suppressed.

Furthermore, according to the above-mentioned arrangement, the outputsignal from the detection pen in the image display period is taken in bythe clamp circuit which operates in synchronization with the imagedisplay period. Further the output signal from the detection pen in thesegment electrode scanning period is taken in by the clamp circuit whichoperates in synchronization with the segment electrode scanning period.Meanwhile, the output signal from the detection pen in the commonelectrode scanning period is taken in by the clamp circuit whichoperates in synchronization with the common electrode scanning period.

Thus the image display voltage inversion pulse, x-coordinate detectionpulse, and the y-coordinate detection pulse contained in a time seriesform in the output signal from the detection pen can be prevented frominterfering with each other in the x-coordinate detection circuit andthe y-coordinate detection circuit.

It is a second object of the present invention to provide adisplay-integrated type tablet device capable of increasing coordinatedetection accuracy by preventing detection of induction noise with thedetection electrode of the detection pen.

In order to achieve the above-mentioned second object, there is provideda display-integrated type tablet device including a display panel whichhas a display material interposed between segment electrodes and commonelectrodes crossing each other at right angles and is driven by a dutyratio control type drive method, a detection pen having at a tip of thedetection pen an electrode electrostatically coupled with the segmentelectrodes and the common electrodes of the display panel, a segmentelectrode drive circuit for driving the segment electrodes, a commonelectrode drive circuit for driving the common electrodes, a displaycontrol circuit for displaying an image on the display panel bycontrolling the segment electrode drive circuit and the common electrodedrive circuit in a period of displaying the image, a detection controlcircuit for controlling the segment electrode drive circuit tosequentially scan the segment electrodes of the display panel byapplying a scanning voltage successively to the segment electrodes andcontrolling the common electrode drive circuit to sequentially scan thecommon electrodes by applying a scanning voltage successively to thecommon electrodes in a coordinate detection period, an x-coordinatedetection circuit for detecting an x-coordinate value designated on thedisplay panel by the tip of the detection pen according to an outputsignal generating timing of the detection pen and a scanning timing ofthe segment electrodes, and a y-coordinate detection circuit fordetecting a y-coordinate value designated on the display panel by thetip of the detection pen according to an output signal generating timingof the detection pen and a scanning timing of the common electrodes, thedisplay-integrated type tablet device comprising a correction voltagegeneration means for generating a correction voltage for canceling avoltage induced at an electrode closer to the detection pen and placedin an upper position attributed to a scanning voltage applied to anelectrode farther to the detection pen and placed in a lower position,the electrodes being one and the other of the segment electrode and thecommon electrode, thereby canceling the voltage induced at the electrodeplaced in the upper position by applying the correction voltagegenerated by the correction voltage generation means to the electrodeplaced in the upper position at least in a period when the electrodeplaced in the lower position is scanned.

It is preferable that the correction voltage generation means comprises:a correction signal generation circuit which stores, in an internalmemory, digital correction data representing an inverted waveform of awaveform of the signal induced at the upper electrode attributed to thescanning voltage applied to the lower electrode, and reads thecorrection data to output the correction data as a correction signal;and a correction voltage generation circuit which receives thecorrection signal from the correction signal generation circuit andgenerates the correction voltage in an analog form based on thecorrection signal.

Further, it is preferable that each of the segment electrode drivecircuit and the common electrode drive circuit comprises a shiftregister, and the correction voltage generation means comprises: acorrection signal generation circuit which takes in a shift data signalfor starting scanning when it is input to an input terminal of a firststage of the shift register of either one of the segment electrode drivecircuit and the common electrode drive circuit for scanning the lowerelectrodes and takes in a pulse signal output from an output terminal ofa final stage of the shift register at the time of ending the scanningto generate a binary correction signal having a pulse widthsubstantially equal to the scanning period of the lower electrodes basedon the shift data signal and the pulse signal; and a correction voltagegeneration circuit which receives the correction signal from thecorrection signal generation circuit to generate the correction voltagein an analog form based on the correction signal.

Furthermore, it is preferable that the display-integrated type tabletdevice comprises an auxiliary electrode facing the lower electrodes,wherein the correction voltage generation means comprises: a correctionsignal generation circuit which detects a voltage induced at theauxiliary electrode attributed the scanning voltage applied to the lowerelectrode and outputs the voltage as a correction signal; and acorrection voltage generation circuit which receives the correctionsignal from the correction signal generation circuit to generate thecorrection voltage based on an inversion signal of the correctionsignal.

Moreover, it is preferable that the correction voltage generation meanscomprises: a correction signal generation circuit which detects acurrent flowing through the upper electrode at the time when thescanning voltage is applied to the lower electrode and outputs thecurrent as a correction signal; and a correction voltage generationcircuit which receives the correction signal from the correction signalgeneration circuit to generate the correction voltage based on thecorrection signal.

Also, it is preferable that the correction voltage generation meanscomprises: a correction signal generation circuit which detects avoltage at the upper electrode at the time when the scanning voltage isapplied to the lower electrode and outputs the voltage as a correctionsignal; and a correction voltage generation circuit which receives thecorrection signal from the correction signal generation circuit togenerate the correction voltage based on the correction signal.

According to the above-mentioned arrangement, when the electrode placedin the lower position farther from the detection pen out of the segmentelectrode and the common electrode constituting the display panel isscanned, a voltage is induced at the electrode in the upper position dueto the scanning voltage applied to the electrode in the lower position.

In the above case, a correction voltage such that it cancels the voltageinduced at the upper electrode is generated by the correction voltagegeneration means to be applied to the upper electrode. Therefore, thevoltage induced at the upper electrode due to the scanning voltageapplied to the lower electrode is canceled.

As a result, no induction noise due to the voltage induced at the upperelectrode attributed to the scanning voltage applied to the lowerelectrode is superimposed on the output signal which is output throughinduction at the electrode of the detection pen electrostaticallycoupled with the segment electrode and the common electrode in theperiod when the lower electrode is scanned.

According to the aforementioned arrangement, digital correction datarepresenting the waveform of an inversion signal of the voltage inducedat the upper electrode attributed to the scanning voltage applied to thelower electrode is read from the internal memory of the correctionsignal generation circuit and transmitted as a correction signal to thecorrection voltage generation circuit. Then the correction voltage in ananalog form is generated based on the correction signal from thecorrection signal generation circuit by the correction voltagegeneration circuit at least in the period when the lower electrode isscanned, and the correction voltage is then applied to the upperelectrode.

Thus the voltage induced at the upper electrode attributed to thescanning voltage applied to the lower electrode is canceled.

According to the aforementioned arrangement, the correction signalgeneration circuit takes in a shift data signal for starting thescanning as input to the input terminal of the first stage of the shiftregister of either of the segment electrode drive circuit or the commonelectrode drive circuit for scanning the lower electrodes as well as apulse signal output from the output terminal of the final stage of theabove-mentioned shift register at the time of ending the scanning. Basedon the shift data signal and the pulse signal, a binary correctionsignal having a pulse width substantially equal to the scanning periodof the lower electrodes is generated. Then the generated binarycorrection signal is transmitted to the correction voltage generationcircuit.

Then the correction voltage generation circuit generates theaforementioned analog correction voltage based on the correction signalfrom the correction signal generation circuit at least in the periodwhen the lower electrode is scanned, and the correction voltage isapplied to the upper electrode.

Thus the voltage induced at the upper electrode attributed to thescanning voltage applied to the lower electrode is canceled.

According to the aforementioned arrangement, the correction signalgeneration circuit detects a voltage induced at the auxiliary electrodefacing the lower electrodes attributed to the scanning voltage appliedto the lower electrode, and the voltage is transmitted as a correctionsignal to the correction voltage generation circuit.

Then the correction voltage generation circuit generates theaforementioned correction voltage based on the inversion signal of thecorrection signal from the correction signal generation circuit at leastin the period when the lower electrode is scanned, and the correctionvoltage is applied to the upper electrode.

Thus the voltage induced at the upper electrode attributed to thescanning voltage applied to the lower electrode is canceled.

According to the aforementioned arrangement, the correction signalgeneration circuit detects the current through the upper electrode atthe time when the scanning voltage is applied to the lower electrode,and a signal representing the current is transmitted as a correctionsignal to the correction voltage generation circuit.

Then the correction voltage generation circuit generates the correctionvoltage based on the correction signal from the correction signalgeneration circuit at least in the period when the lower electrode isscanned, and the correction voltage is applied to the upper electrode.

Thus the voltage induced at the upper electrode attributed to thescanning voltage applied to the lower electrode is canceled.

According to the aforementioned arrangement, the correction signalgeneration circuit detects the voltage at the upper electrode at thetime when the scanning voltage is applied to the lower electrode, andthe voltage is transmitted as a correction signal to the correctionvoltage generation circuit.

Then the correction voltage generation circuit generates the correctionvoltage based on the correction signal from the correction signalgeneration circuit at least in the period when the lower electrode isscanned, and the correction voltage is applied to the upper electrode.

Thus the voltage induced at the upper electrode attributed to thescanning voltage applied to the lower electrode is canceled.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 is a block diagram of a coordinate detection system of adisplay-integrated type tablet device in accordance with an embodimentof the present invention;

FIG. 2 is a timing chart of periodical signals output from a controlcircuit as shown in FIG. 1;

FIGS. 3(a) and 3(b) are exemplified circuit diagrams of a detectionsignal clamp circuit and an image display inversion signal clamp circuitas shown in FIG. 1;

FIGS. 4(a), 4(b), 4(c) and 4(d) are exemplified circuit diagrams of peakhold circuits as a detection and hold circuit as shown in FIG. 1;

FIGS. 5(a) and 5(b) are exemplified circuit diagrams of average valuedetection and hold circuits as a detection and hold circuit as shown inFIG. 1;

FIG. 6 is a graph showing an input-output characteristic of thedetection and hold circuit having an offset incorporation circuit;

FIG. 7 is a chart showing the waveforms of output signals of eachcircuit in the stage for removing noise from the x-coordinate detectionsignal;

FIG. 8 is a diagram for explaining a logic circuit for obtaining anx-coordinate value and a y-coordinate value from an x-coordinate signaland a y-coordinate signal;

FIG. 9 is a block diagram of a coordinate detection system differentfrom that shown in FIG. 1;

FIGS. 10(a) and 10(b) are a diagram showing the equivalent circuits of asegment electrode drive circuit and a common electrode drive circuit anda power supply system for use in a display-integrated type tablet deviceof the present invention;

FIG. 11 is a diagram showing an example of the power supply system asshown in FIG. 10;

FIG. 12 is a diagram of an exemplified correction signal generationcircuit and an exemplified correction voltage generation circuit asshown in FIG. 11;

FIGS. 13 is a chart of exemplified correction signal and correctionvoltage;

FIGS. 14(a), 14(b) and 14(c) are diagrams of exemplified power supplysystems different from that shown in FIG. 11;

FIG. 15 is a block diagram of a display-integrated type tablet device;

FIG. 16 is a diagram for explaining the image display period and thecoordinate detection period of the display-integrated type tablet deviceas shown in FIG. 15;

FIGS. 17(a), 17(b) and 17(c) are diagrams showing the floatingcapacitance between the segment electrode X or the common electrode Yand the detection pen, a detection signal obtained by the detection pen,and a binary signal of the detection signal in the display-integratedtype tablet device as shown in FIG. 15;

FIG. 18 is a timing chart of the segment electrode scanning signalapplied to the segment electrode X in the x-coordinate detection period,a voltage at the common electrode, and an exemplified detection signalcontaining induction noise peaks in the display-integrated type tabletdevice as shown in FIG. 15;

FIG. 19 is a diagram of equivalent circuits of a segment electrode drivecircuit and a common electrode drive circuit in the display-integratedtype tablet device as shown in FIG. 15;

FIG. 20 is a timing chart of a segment electrode scanning signal and acommon electrode scanning signal in the display-integrated type tabletdevice as shown in FIG. 15;

FIG. 21 is a diagram for explaining a relation in position between theLCD panel and the detection pen;

FIG. 22 is an equivalent circuit diagram of the LCD panel;

FIG. 23 is a simplified equivalent circuit diagram of the circuit asshown in FIG. 22;

FIG. 24 is a diagram of a condition of the circuit shown in FIG. 23where the segment electrode scanning is advanced by one clock pulse;

FIG. 25 is a diagram of a condition of the circuit shown in FIG. 23 atthe time of starting the scanning of the segment electrode;

FIG. 26 is a diagram of a condition of the circuit shown in FIG. 23 atthe time of ending the scanning of the segment electrode;

FIG. 27 is a chart of an exemplified x-coordinate detection signal;

FIG. 28 is a timing chart of waveforms in each part of FIG. 1;

FIG. 29 is a diagram of an exemplified circuit effecting a mutingcontrol based on an output of a delay element; and

FIG. 30 is a chart showing waveforms in each part of FIG. 29.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following describes the present invention based on its severalembodiments with reference to the attached drawings.

<First embodiment>

FIG. 1 is a block diagram of a coordinate system of thedisplay-integrated type tablet device of the first embodiment.

Referring to FIG. 1, there are included an LCD panel 1, a transparentprotection panel 16, a detection electrode 13 of a detection pen 8, andan amplifier 9 built in the detection pen 8. There are further includeda detection signal clamp circuit 20, a detection signal amplifier 21, anX-coordinate signal clamp circuit 22, an X-coordinate signal amplifier23, a detection and hold circuit 24, a delay element 25, an adder 26,and a comparator 27. There are still further included a Y-coordinatesignal clamp circuit 28, a Y-coordinate signal amplifier 29, acomparator 30, an image display inversion signal clamp circuit 32, adetection and hold circuit 33, and a control circuit 7.

The control circuit 7 outputs a coordinate detection period signal a, ay-coordinate detection period signal b, an x-coordinate detection periodsignal c, and an image display period signal d according to a timing asshown in FIG. 2. Among the above-mentioned signals, the image displayperiod signal d may be a signal d' which is made active only at theimage display voltage inversion time, or may be a signal d" which ismade active at the image display voltage inversion time just before they-coordinate detection period signal b becomes active. A detectionsignal induced at the detection electrode 13 of the detection pen 8 isshown by a waveform e.

The detection signal e which was induced at the detection electrode 13of the detection pen 8 and amplified in the amplifier 9 is input to thedetection signal clamp circuit 20 and the image display inversion signalclamp circuit 32 by way of a cable. The detection signal clamp circuit20 and the image display inversion signal clamp circuit 32 are eachcomposed of two resistors 43 and 44, one capacitor 42, and onevoltage-controlled switch element 41 such as a transistor or afield-effect transistor as shown in FIG. 3(a). Also, the above clampcircuits 20 and 32 may be each constituted as shown in FIG. 3(b) andcombined with an operational amplifier.

Referring to FIGS. 3(a) and 3(b), the voltage-controlled switch element41 turns OFF to increase its impedance when an active control voltage of5 V is input to its control terminal. The switch element 41 turns ONwhen a non-active control voltage of 0 V is input to the terminal.

While the control voltage is at 0 V, the voltage-controlled switchelement 41 continues to be ON, and therefore a voltage V₂ as shown inFIG. 3(a) or a current i₂ as shown in FIG. 3(b) becomes "0", when theinput voltage V₁ is utterly interrupted from the output side to allowthe capacitor 42 to be charged or discharged. When the control voltagechanges from 0 V to 5 V, the interruption between the input side and theoutput side is released, and the voltage V₂ or the current i₂ rises upfrom "0" by the charge in the capacitor 42 charged just in advance.

As shown in FIG. 2, the aforementioned LCD image display polarityinversion pulse, the y-coordinate detection pulse, and the x-coordinatedetection pulse are timely close to each other. Furthermore, due to ahigh-pass filter effect attributed to a capacitive coupling between thedetection electrode 13 of the detection pen 8 and the LCD panel 1, delayof low-frequency component of the previous signal is disadvantageouslysuperimposed on the main signal component which is desired to beextracted. However, since the detection signal clamp circuit 20, thex-coordinate detection signal clamp circuit 22, the y-coordinatedetection signal clamp circuit 28 and the image display inversion signalclamp circuit 32 are electrically equivalent to the circuit as shown inFIGS. 3(a) or 3(b), the input side and the output side in each clampcircuit 20, 22, 28 and 32 are electrically separated from each otherwhen the voltage-controlled switch element 41 is ON. In that time, theoutput side of the capacitor 42 is fixed to the ground potential. Then,when the voltage-controlled switch element 41 changes from ON to OFF,the output signal V2 starts from 0 V due to the charge effect of thecapacitor 42 and only the high-frequency component of the input signalV1 passes through. Thus, clamping operation is executed. In the case,the input and output signals of the detection signal clamp circuit 20,the output signal of the x-coordinate detection signal clamp circuit 22and the output signal of the y-coordinate detection signal clamp circuit28 are shown in FIG. 28.

To the control terminal of the voltage-controlled switch element 41 inthe image display inversion signal clamp circuit 32 is applied the imagedisplay period signal d which becomes active only in the image displayperiod as shown in FIG. 2. As a result, only the LCD image displaypolarity inversion pulse of the detection signal e as shown in FIG. 2 ispassed to be transmitted to the detection and hold circuit 33 in thenext stage.

The detection and hold circuit 33 may be a peak hold circuit fordetecting a peak value, or may be an average value detection and holdcircuit including an integration function. Exemplified peak holdcircuits are shown in FIGS. 4(a), 4(b), 4(c), and 4(d), and exemplifiedaverage value detection and hold circuits are shown in FIGS. 5(a) and5(b).

In the above-mentioned detection and hold circuit 33, a DC voltagecorresponding to the level of the LCD image display polarity inversionpulse is obtained and applied as a threshold voltage to an inputterminal of the comparator 27 and the comparator 30 in the next stage.

Meanwhile, to the control terminal of the voltage-controlled switchelement 41 in the detection signal clamp circuit 20 is applied adetection period signal a which becomes active only in the coordinatedetection period as shown in FIG. 2. As a result, only the detectionsignal including the y-coordinate detection pulse and the x-coordinatedetection pulse in the coordinate detection period is passed to be inputto the detection signal amplifier circuit 21 in the next stage. Then thedetection signal amplified in the detection signal amplifier circuit 21is transmitted to the X-coordinate signal clamp circuit 22 and theY-coordinate signal clamp circuit 28.

Since the y-coordinate detection period signal b as shown in FIG. 2 isapplied to the control terminal of the electronic control switch elementof the Y-coordinate signal clamp circuit 28, only the y-coordinatedetection signal is passed. The signal is amplified in the Y-coordinatesignal amplifier circuit 29 and supplied to the comparator 30.

In the above case, there is already applied a threshold voltageproportional to the magnitude of the LCD polarity inversion signal fromthe detection and hold circuit 33 to the other input terminal of thecomparator 30 as described hereinbefore. Therefore, the comparator 30outputs a y-coordinate signal having a positive pulse only when itdetects a y-coordinate detection signal exceeding the threshold value.

In the same manner, the x-coordinate detection period signal c as shownin FIG. 2 is applied to the control terminal of the electronic controlswitch of the X-coordinate signal clamp circuit 22, and therefore onlythe x-coordinate detection signal is passed. Then the signal isamplified in the X-coordinate signal amplifier 23 and supplied to apositive input terminal of the adder 26 as a synthesis circuit and thedetection and hold circuit 24.

The detection and hold circuit 24 has the same construction as that ofthe detection and hold circuit 33, and therefore a DC voltage accordingto the level of the x-coordinate detection signal is output.

It is noted that the detection and hold circuit 24 has an offsetincorporation circuit having a deadband as shown in FIG. 6.

The output of the detection and hold circuit 24 is delayed by the delayelement 25 and applied to a negative input terminal of the adder 26 tobe added to the original x-coordinate detection signal.

FIG. 7 shows output signals output from the detection and hold circuit24, the delay element 25, and the adder 26 in the case where anx-coordinate detection signal as shown in FIG. 27 is input to thedetection and hold circuit 24. After detecting the x-coordinatedetection pulse, the output signal from the adder 26 becomes a signal inwhich the noise component in the trailing portion of the pulse issuppressed in amplitude by a voltage corresponding to the level of theoutput signal from the detection and hold circuit 24 (i.e., thex-coordinate detection peak level).

Referring to FIG. 29, the output of a delay element 25 is converted intoa binary signal by a comparator 31. The binary signal controls a mutingcircuit 32 as a digital control signal. As a result, the dip componentfollowing the peak in the output signal of the x-coordinate signalamplifier 23 can be ignored as understood from waveforms in FIG. 30.

The output of the adder 26 is input to an input terminal of thecomparator 27. To the other input terminal of the comparator 27 isalready applied a threshold voltage proportional to the magnitude of theLCD polarity inversion signal from the detection and hold circuit 33.Therefore, the comparator 27 outputs an x-coordinate signal having apositive pulse only when it detects an x-coordinate detection pulseexceeding the threshold value.

The x-coordinate pulse of the x-coordinate signal output from thecomparator 27 and the y-coordinate pulse of the y-coordinate signaloutput from the comparator 30 are digital pulses where the position ofthe detection pen 8 on the tablet is reflected on a time. Therefore, byusing a logic circuit such that, with a counter to be reset at therise-time of the x-coordinate detection period signal c and they-coordinate detection period signal b of the control circuit 7, thecounting in the counter is stopped by the x-coordinate pulse of thex-coordinate signal or the y-coordinate pulse of the y-coordinatesignal, the time of the x-coordinate pulse or the y-coordinate pulse canbe obtained as a count value to consequently allow the positiondesignated by the detection pen 8 on the LCD panel 1 to be known. FIG. 8shows an example of such a logic circuit.

In the present embodiment as described above, the LCD image displaypolarity inversion pulse, the y-coordinate detection pulse, and thex-coordinate detection pulse are separated from the detection signalinduced at the detection electrode 13 of the detection pen 8 by means ofthe detection signal clamp circuit 20, the image display inversionsignal clamp circuit 32; the X-coordinate signal clamp circuit 22, andthe Y-coordinate signal clamp circuit 28. With the above-mentionedarrangement, there can be input the LCD image display polarity inversionpulse to the detection and hold circuit 33, the x-coordinate detectionpulse to the X-coordinate signal amplifier 23, and the y-coordinatedetection pulse to the Y-coordinate signal amplifier 29 without anymutual interference.

Furthermore, a DC voltage corresponding to the level of the LCD imagedisplay polarity inversion pulse is obtained by the detection and holdcircuit 33. Then the DC voltage is made to serve as a reference voltagein obtaining a binary pulse of the x-coordinate detection pulse or they-coordinate detection pulse in the comparators 27 and 28.

In the above case, the LCD image display polarity inversion pulsechanges according to the distance between the detection electrode 13 ofthe detection pen 8 and the segment electrode X and the common electrodeY. Therefore, the aforementioned reference voltage is also changedaccording to the above-mentioned distance. Consequently, theaforementioned reference voltage changes according to the distance. Theabove means that both the detection signal voltage and the referencevoltage input to the comparators 27 and 28 change according to thedistance between the detection pen 8 and the segment electrode X and thecommon electrode Y. As a result, the x-coordinate detection signal andthe y-coordinate detection signal output from the comparators 27 and 28become signals which are not dependent on the above-mentioned distance.

Furthermore, the detection and hold circuit 24, the delay element 25,and the adder 26 for detecting an x-coordinate detection pulse areincorporated to the x-coordinate detection circuit 10 to add thex-coordinate detection signal which has passed through the detection andhold circuit 24 and the delay element 25 to the original x-coordinatedetection signal. As a result, when an x-coordinate detection pulsehaving the maximum intensity is detected, the subsequent noise componentcan be suppressed.

<Second embodiment>

FIG. 9 is a block diagram of a coordinate detection system in thedisplay-integrated type tablet device of the second embodiment. It isnoted that the same parts and components as those in FIG. 1 are denotedby the same numerals, and no description is provided therefor herein.

Since the detection signal received from the common electrode locatedcloser to the detection pen 8 has a high signal-to-noise ratio, ay-coordinate pulse having a sufficiently reduced error can be obtainedeven with a fixed threshold value. Therefore, by taking advantage of theabove-mentioned fact, the circuit scale of the coordinate detectionsystem of the first embodiment can be simplified.

It is noted that the second embodiment differs from the first embodimentin the following point. The point is that there is used a DC voltageproportional to the level of the y-coordinate detection pulse which isgenerated before the generation of the x-coordinate detection signal asa reference for the comparator 27 for x-coordinate detection signal.Therefore, the image display inversion signal clamp circuit 32 and thedetection and hold circuit 33 for obtaining the DC voltage proportionalto the level of the LCD image display polarity inversion pulse areeliminated, and a detection and hold circuit 31 is incorporated insteadin the second embodiment.

The present invention is not limited to the above-mentioned arrangement.There is no problem even when the vertical positions of the commonelectrodes and the segment electrodes of the LCD panel 1 are exchanged,the sequence in time of the y-coordinate detection period and thex-coordinate detection period are exchanged, and the x-coordinatedetection circuit and the y-coordinate detection circuit as shown inFIG. 9 are exchanged.

As apparent in the above description, the display-integrated type tabletdevice of the present invention scans the upper electrodes locatedcloser to the detection pen out of the segment electrode and the commonelectrode under the control of the detection control circuit in thefirst scanning period to detect and hold the voltage variation quantityof the upper electrodes according to the output signal from thedetection pen in the above time and output a reference signalcorresponding to the above-mentioned voltage variation quantity. Thenthe coordinate detection circuit relevant to the lower electrodes out ofthe aforementioned x-coordinate detection circuit and the y-coordinatedetection circuit detects a coordinate detection pulse contained in theoutput signal from the detection pen based on the aforementionedreference signal. Therefore, the detection level of the coordinatedetection pulse relevant to the lower electrode susceptible to externalnoise can be changed based on the voltage variation quantity at the timeof scanning the upper electrodes.

Therefore, according to the present invention, the variation of theoutput signal from the detection pen attributed to the change of thedistance between the detection electrode of the detection pen and thesegment electrode and the common electrode can be appropriatelycorrected to allow a high-accuracy coordinate detection to be achieved.

Furthermore, the display-integrated type tablet device of the presentinvention detects and holds the voltage variations of the segmentelectrode and the common electrode by means of the detection circuitaccording to the output signal from the detection pen to output areference signal corresponding to the above-mentioned voltage variationin the image display period. Then the x-coordinate detection circuit andthe y-coordinate detection circuit detect the coordinate detection pulsecontained in the output signal from the detection pen based on theabove-mentioned reference signal. Therefore, in both the x-coordinatedetection circuit and the y-coordinate detection circuit, the variationof the output from the detection pen can be corrected moreappropriately.

Furthermore, the display-integrated type tablet device of the presentinvention detects and holds the coordinate detection pulse contained inthe output signal from the detection pen by means of the detectioncircuit, delays the resulting pulse for a specified period by means ofthe delay circuit, and synthesizes the detection signal from the delaycircuit and the detection signal from the detection pen by means of thesynthesis circuit. Therefore, the noise component following the peak ofthe coordinate detection pulse in the detection signal can be suppressedto allow the coordinate detection accuracy to be increased.

Furthermore, by virtue of the three clamp circuits operating insynchronization respectively with the image display period, segmentelectrode scanning period, and the common electrode scanning period, thedisplay-integrated type tablet device of the present invention takes inthe output signal from the detection pen in the aforementioned threeperiods. Therefore, the image display voltage inversion pulse, thex-coordinate detection pulse, and the y-coordinate detection pulse canbe prevented from interfering with each other inside the x-coordinatedetection circuit and the y-coordinate detection circuit to allow thecoordinate detection accuracy to be increased.

<Third embodiment>

It is noted that the segment electrode is placed under the commonelectrode in the display-integrated type tablet device of a thirdembodiment of the present invention. It is further noted that the commonelectrode drive circuit, segment electrode drive circuit, switchingcircuit, display control circuit, detection control circuit, controlcircuit, detection pen, amplifier, x-coordinate detection circuit,y-coordinate detection circuit, DC power supply circuit, and the like ofthe display-integrated type tablet device of the present embodiment arethe same as those of the display-integrated type tablet device as shownin FIG. 15.

FIG. 10(a) is a diagram showing the LCD panel, segment electrode drivecircuit, common electrode drive circuit, and power supply system of thepresent third embodiment.

The common electrode drive circuit 2 and the segment electrode drivecircuit 3 shown in FIG. 10(a) have the same equivalent circuits as thoseof the common electrode drive circuit 102 and the segment electrodedrive circuit 103 as shown in FIG. 19. It is noted that the power to thecommon electrode drive circuit 2 and the segment electrode drive circuit3 of the present third embodiment is supplied in the following manner.

The DC power supply circuit 6 supplies bias power sources V₀ through V₅in the same manner as the DC power supply circuit 112 shown in FIG. 15.The bias power source V₅ from the DC power supply circuit 6 is suppliedto each on-resistor r_(c5) of the common electrode drive circuit 2 andto each on-resistor r_(s5) of the segment electrode drive circuit 3.Meanwhile, the bias power source V₁ from the DC power supply circuit 6is supplied to a correction voltage generation circuit 5.

The correction voltage generation circuit 5 generates a correctionvoltage V₁ ' based on the correction signal input from a correctionsignal generation circuit 4. The correction voltage V₁ ' is a voltagefor correcting a shift of a voltage generated at the common electrode Yplaced in the upper position attributed to the scanning voltage V₅applied to the segment electrode X in the time of scanning the segmentelectrode X placed in the lower position, the voltage shifting towardthe scanning voltage V₅ as shown by a waveform (a) in FIGS. 13 (same asthe waveform (a) in FIG. 18). Therefore, the correction voltage has awaveform as shown in FIG. 13(b) which is an inversion in polarity of thewaveform (a) shown in FIG. 13.

The correction voltage V₁ ' from the correction voltage generationcircuit 5 is supplied to each on-resistor r_(c1) of the common electrodedrive circuit 2 and to each on-resistor r_(s1) of the segment electrodedrive circuit 3.

The reason why the correction voltage V₁ 40 is supplied not only to theon-resistor r_(c1) of the common electrode drive circuit 2 but also tothe on-resistor r_(s1) of the segment electrode drive circuit 3 is asfollows. The above arrangement is adopted so as to prevent, in thex-coordinate detection period, the generation of any voltage differencebetween all the common electrodes Y to which the correction voltage V₁ 'is applied in the non-scanning state and the segment electrode X(segment electrodes X₁, X₅, . . . , X_(m) in FIG. 10(a)) in the scanningstate.

It is noted that the difference between the non-scanning voltage V₁ andthe correction voltage V₁ ' is small, and there is almost no problemeven when the non-scanning voltage V₁ is supplied to each on-resistorr_(s1) of the segment electrode drive circuit 3.

The correction signal output from the correction signal generationcircuit 4 may be an analog signal or a digital signal. When thecorrection signal is an analog signal, the correction voltage generationcircuit 5 is required to be composed of, for example, an operationalamplifier to merely amplify the power of the analog correction signal.When the correction signal is a digital signal, the correction voltagegeneration circuit 5 is required to be composed of a digital-to-analogconverter or a resistor and a capacitor to effect an approximationcorrection through a waveform shaping of the input digital correctionsignal.

The following describes examples of the correction signal generationcircuit 4 and the correction voltage generation circuit 5.

Referring to FIG. 10(b), in the correction signal generation circuit 4of the present third embodiment, correction data formed by expressingthe waveform of the correction voltage V₁ ' (reference voltage=0) by adigital value (the correction data obtained through an analog-to-digitalconversion of the analog waveform (b) in FIG. 13) is preliminarilystored in an internal memory 410 such as a ROM (Read Only Memory) or aRAM (Random Access Memory). When data processing enters into thex-coordinate detection period set by the control circuit 107 forscanning the segment electrode X placed in the lower position, thecorrection signal generation circuit 4 reads out the correction datafrom the internal memory 410 and transmits the signal as a correctionsignal to the correction voltage generation circuit 5.

Then the correction voltage generation circuit 5 subjects the correctionsignal (correction data) from the correction signal generation circuit 4to a digital-to-analog conversion with a digital-to-analog (D/A)convertor 514 and superimpose the resulting signal on the bias powersource V₁ from the DC power supply circuit 6 by an adder 515 toconsequently obtain the correction voltage V₁ ' having a waveform (b) inFIG. 13.

In the present third embodiment as described above, the digital data(correction data) of the waveform of the correction voltage V₁ ' ispreliminarily stored in the internal memory 410 of the correction signalgeneration circuit 4. Then in the x-coordinate detection period, thecorrection data is subjected to a digital-to-analog conversion in theD/A convertor 514 of the correction voltage generation circuit 5 to besuperimposed on the bias power source V₁ to thereby obtain thecorrection voltage V₁ '. Then the obtained correction voltage V₁ ' isapplied as a non-scanning voltage to the common electrode Y in thenon-scanning state.

Therefore, according to the third embodiment, the voltage induced at thecommon electrode Y attributed to the scanning voltage V₅ applied to thesegment electrode X is canceled in the x-coordinate detection period todetect no induction noise at the detection electrode of the detectionpen to thereby allow the coordinate detection accuracy to be increased.

As a simple modification of the third embodiment, a digital value of abinary signal waveform having a pulse width substantially equal to theentire x-coordinate detection period as shown by a waveform (c) in FIG.13 is stored in the internal memory 410 of the correction signalgeneration circuit 4. Then the correction voltage generation circuit 5converts the binary signal waveform (c) shown in FIG. 13 into a voltagewaveform similar to a waveform (b) shown in FIG. 13 by means of acircuit composed of a combination of a capacitance C and a resistance R(not shown) (referred to as the "CR circuit" hereinafter). The resultingsignal is then superimposed on the bias power source V_(l) from the DCpower supply circuit 6 to thereby obtain the correction voltage V₁ 'having a waveform (d) shown in FIG. 13.

Strictly describing the above case, the correction voltage V₁ ' differsin each display-integrated type tablet device, and therefore it isrequired to store different correction data for each display-integratedtype tablet device. However, in practical cases, it is sufficient toobtain a distribution of the correction voltages V₁ ' of all thedisplay-integrated type tablet devices and store an average correctionvoltage V₁ ' as a correction value in the internal memory 410.

It is noted that, when a high-accuracy correction voltage V_(l) ' isnecessary, it is required to store correct correction data in eachdisplay-integrated type tablet device. In such a case, it is sufficientto store correction data corresponding to each common electrode Y in aflash memory or an E² ROM (Electrically Erasable ROM) or the like.

<Fourth embodiment>

Referring to FIG. 11, a fourth embodiment differs from the thirdembodiment in the correction signal generation circuit 404. In thecorrection signal generation circuit 404 of the fourth embodiment, shiftdata so from the switching circuit 104 (refer to FIG. 15) and a pulsesignal seio2 (refer to FIG. 15) output from an output terminal EIO2 ofthe segment electrode drive circuit 3 are taken in as shown in FIG. 11,and a correction signal generated based on the shift data so and thepulse signal seio2 is output to the correction voltage generationcircuit 5.

In more detail, there is generated a correction signal as shown by awaveform (g) in FIG. 13 having a rectangular waveform of which logiclevel becomes "H" at the rise-time of the shift data so as shown by awaveform (e) in FIG. 13 and becomes "L" at the rise-time of the pulsesignal seio2 as shown by a waveform (f) in FIG. 13.

Then the correction voltage generation circuit 5 outputs a correctionvoltage V₁ ' having a waveform as shown by a waveform (d) in FIG. 13based on the correction signal having the above-mentioned rectangularwaveform from the correction signal generation circuit 404.

The correction signal generation circuit 404 which generates theabove-mentioned correction signal and the correction voltage generationcircuit 5 which generates the correction voltage V₁ ' are each requiredin practice to be composed of a combination of a simple digital circuitsuch as a flip-flop circuit and a CR circuit, or a CR circuit as shownin FIG. 12.

When a higher accuracy is required, it is required that a variableresistor is incorporated to the CR circuit as shown in FIG. 12 to makethe CR circuit adjustable for each common electrode Y.

According to the present fourth embodiment as described above, thecorrection signal having a rectangular waveform is generated in thecorrection signal generation circuit 404 based on the shift data soinput to the segment electrode drive circuit 3 and the pulse signalseio2 output from the segment electrode drive circuit 3. Therefore, thecorrection voltage V₁ ' can be generated easily without storing anycorrection data into the internal memory nor providing any additionalparts to allow a voltage induced at the common electrode Y attributed tothe scanning voltage V₅ applied to the segment electrode X placed in thelower position to be removed.

<Fifth embodiment>

Referring to FIG. 14(a), the fifth embodiment differs from the third andfourth embodiments only in a correction signal generation circuit 414.As shown in FIG. 14(a), the correction signal generation circuit 414 ofthe fifth embodiment outputs to the correction voltage generationcircuit 5 a correction signal generated based on a voltage induced inthe time of scanning the segment electrode X at an auxiliary electrodeY₀ provided in addition to the segment electrode X and the commonelectrode Y on the LCD panel 1.

In more detail, the auxiliary electrode Y₀ is provided against thesegment electrode X in parallel with a common electrode Y₁ outside thecommon electrode Y₁ in the same plane as that of the common electrodes Yplaced in the upper position. When the scanning voltage V₅ is applied tothe segment electrode X in the x-coordinate detection period, a voltageis induced at the auxiliary electrode Y₀ in the same manner as thecommon electrode Y to generate a voltage having a waveform (a) shown inFIG. 13 (note that the reference voltage is not V₁).

The voltage induced at the auxiliary electrode Y₀ is detected by thecorrection signal generation circuit 414 including an operationalamplifier 415 and a gate 416 and transmitted as a correction signal tothe correction voltage generation circuit 5.

Then the correction voltage generation circuit 5 including an adder 515inverts the polarity of the correction signal having a waveform similarto the waveform (a) shown in FIG. 13, and superimposes the resultingsignal on the bias power source V₁ from the DC power supply circuit 6 tothereby obtain a correction voltage V₁ ' having a waveform (b) shown inFIG. 13.

The aforementioned auxiliary electrode Y₀ is not necessarily formed inthe same plane as that of the common electrode Y. The auxiliaryelectrode Y₀ may be formed, for example, on the surface opposite fromthe common electrode Y (i.e., the reverse side). In such a case, theauxiliary electrode Y₀ can be formed by attaching a copper tape havingan adhesive layer onto the external surface of the LCD panel 1. Byadopting such an arrangement, the auxiliary electrode Y₀ can be easilyformed afterward on a completely formed LCD panel.

In the fifth embodiment as described above, there is provided thespecial-use auxiliary electrode Y₀ for directly detecting the voltageinduced at the common electrode Y attributed to the scanning voltage V₅applied to the segment electrode X placed in the lower position, andthen the correction voltage V₁ ' is formed based on the voltage inducedat the auxiliary electrode Y₀.

Therefore, the voltage induced at the common electrode Y attributed tothe scanning voltage V₅ applied to the segment electrode X can besecurely removed.

<Sixth embodiment>

Referring to FIG. 14(b), a correction signal generation circuit 554 ofthe sixth embodiment detects via a resistor 591 the value of a currentflowing through the common electrode Y placed in the upper position inthe x-coordinate detection period when the segment electrode X placed inthe lower position is scanned, and outputs the detected signal as acorrection signal to the correction voltage generation circuit 515. Thecorrection signal generation circuit 554 is composed of an amplifier 555and a gate 556. The correction voltage generation circuit 510 iscomposed of an adder 515.

The induction noise peaks F and R as shown in a waveform (b) in FIG. 18are generated due to a current flowing through the common electrode Y.Therefore, the current flowing through the common electrode Y has thesame waveform as the voltage waveform (a) shown in FIG. 13. With theabove-mentioned arrangement, the correction voltage V₁ ' can begenerated by the correction voltage generation circuit 510 in the samemanner as in the fifth embodiment based on the correction signal whichis the current signal of the common electrode Y.

<Seventh embodiment>

Referring to FIG. 14(c), the correction signal generation circuit 614 ofthe present seventh embodiment detects a voltage induced at the commonelectrode Y placed in the upper position attributed to the scanningvoltage V₅ applied to the segment electrode X indirectly from thevoltage at the common electrode Y when the segment electrode X placed inthe lower position is scanned. Then the detected voltage at the commonelectrode Y is output as a correction signal to the correction voltagegeneration circuit 500. In more detail, according to the present seventhembodiment, the correction signal is taken out of one or a plurality ofcommon electrodes Y instead of the auxiliary electrode Y₀ of the fifthembodiment. In addition, the correction signal generation circuit 614includes an operational amplifier 615 and a gate 616.

According to the seventh embodiment as described above, the voltageinduced at the common electrode Y attributed to the scanning voltage V₅applied to the segment electrode X placed in the lower position isdetected indirectly from the common electrode Y to generate thecorrection voltage V₁ ' based on the detected induction voltage.

With the above-mentioned arrangement, the voltage induced at the commonelectrode Y attributed to the scanning voltage V₅ applied to the segmentelectrode X can be removed with a simple device construction as comparedwith the fifth embodiment.

Although the common electrode Y is placed over the segment electrode Xin the LCD panel 1 of each of the aforementioned embodiments, the sameeffect can be obtained when the segment electrode X is placed over thecommon electrode Y. It is noted in the above case that the period wheneach of the correction signal generation circuits 4, 404, 554, 614operates is required to be the y-coordinate detection period when thecommon electrode Y placed in the lower position is scanned.

As apparent in the above description, a display-integrated type tabletdevice of the present invention is provided with a correction voltagegeneration means for generating a correction voltage which cancels thevoltage induced at the electrode placed in the upper position attributedto the scanning voltage applied to the electrode placed in the lowerposition farther from the detection pen out of the segment electrode andthe common electrode constituting the display panel. At least in theperiod when the lower electrode is scanned, the correction voltagegenerated by the correction voltage generation means is applied to theupper electrode to cancel the voltage induced at the upper electrode,and therefore the correct non-scanning voltage is applied to the upperelectrode.

Therefore, in the period when the lower electrode is scanned, no voltageis induced at the detection electrode of the detection pen due to avoltage induced at the upper electrode attributed to the scanningvoltage applied to the lower electrode.

In other words, the present invention can provide a display-integratedtype tablet device capable of increasing the coordinate detectionaccuracy by preventing detection of induction noise at the detectionpen.

Furthermore, in a display-integrated type tablet device of the presentinvention, the correction voltage generation means has a correctionsignal generation circuit and a correction voltage generation circuit.The correction signal generation circuit reads out digital correctiondata representing a waveform which is the inverted voltage of thevoltage induced at the upper electrode attributed to the scanningvoltage applied to the lower electrode as stored in the internal memory,and outputs the data as a correction signal to the correction voltagegeneration circuit. Then the correction voltage generation circuitgenerates the aforementioned analog correction voltage based on thecorrection signal to allow the correction voltage to be generated in asimple manner.

Therefore, the present invention can provide a display-integrated typetablet device capable of increasing the coordinate detection accuracy bypreventing detection of induction noise at the detection pen merely bystoring the correction data in the internal memory and effecting a verysimple processing operation.

Furthermore, in a display-integrated type tablet device of the presentinvention, each of the segment electrode and the common electrode has ashift register, and the aforementioned correction voltage generationmeans has a correction signal generation circuit and a correctionvoltage generation circuit. The correction signal generation circuitgenerates a binary correction signal having a pulse width substantiallyequal to the scanning period of the lower electrodes based on the shiftdata signal input to the input terminal of the first stage of the shiftregister of the drive circuit of either the segment electrode drivecircuit or the common electrode drive circuit for scanning the lowerelectrodes and the pulse signal output from the output terminal of thefinal stage of the above-mentioned shift register at the time of endingthe scanning, and transmits the correction signal to the correctionvoltage generation circuit. The correction voltage generation circuitgenerates the aforementioned analog correction voltage based on thecorrection signal. With the above-mentioned arrangement, the correctionvoltage can be generated in a simple processing operation with the shiftdata signal and the pulse signal which are generally used, withoutproviding any internal memory nor special detection means.

Therefore, the present invention can provide a display-integrated typetablet device capable of increasing the coordinate detection accuracy ina simple processing operation by preventing detection of induction noiseat the detection pen.

Furthermore, a display-integrated type tablet device of the presentinvention is provided with an auxiliary electrode facing the lowerelectrode farther from the detection pen, and the correction voltagegeneration means has a correction signal generation circuit and acorrection voltage generation circuit. The correction signal generationcircuit detects the voltage induced at the auxiliary electrodeattributed to the scanning voltage applied to the lower electrode, andtransmits the voltage as a correction signal to the correction voltagegeneration circuit. The correction voltage generation circuit generatesa correction voltage based on the inversion signal of the correctionsignal to allow the voltage induced at the upper electrode to bedirectly detected for the generation of the correction voltage.

Therefore, the present invention can provide a display-integrated typetablet device capable of further increasing the coordinate detectionaccuracy by securely removing the voltage induced at the upper electrodeattributed to the scanning voltage applied to the lower electrodethereby preventing detection of induction noise at the detection pen.

Furthermore, in a display-integrated type tablet device of the presentinvention, the correction voltage generation means has a correctionsignal generation circuit and a correction voltage generation circuit,and the correction signal generation circuit detects the current throughthe upper electrode taking place when the scanning voltage is applied tothe lower electrode, and transmits the current as a correction signal tothe correction voltage generation circuit. The correction voltagegeneration circuit generates a correction voltage based on thecorrection signal to allow the correction voltage to be generated basedon the current flowing through the upper electrode.

Furthermore, in a display-integrated type tablet device of the presentinvention, the correction voltage generation means has a correctionsignal generation circuit and a correction voltage generation circuit,and the correction signal generation circuit detects the voltage inducedat the upper electrode taking place when the scanning voltage is appliedto the lower electrode, and transmits the voltage as a correction signalto the correction voltage generation circuit. The correction voltagegeneration circuit generates a correction voltage based on thecorrection signal to allow the correction voltage to be generated byindirectly detecting the voltage induced at the upper electrode with asimple device construction.

Therefore, the present invention can provide a display-integrated typetablet device capable of increasing the coordinate detection accuracywith a relatively simple device construction by preventing detection ofinduction noise at the detection pen.

In the forgoing explanation, described is a means for correcting voltageinducted at the upper electrodes in the time of scanning the lowerelectrodes. In addition, it is also important to make induction voltagelower as much as possible. For that purpose, it is necessary to lowerthe resistance values of the on-resistor rc1 of the common drive circuitand the common electrode formed of indium tin oxide. In the LCD panelonly for displaying, the on-resistor of the common drive circuit hasresistance value of about 1-2 KΩ, but in the LCD panel having tabletfunction, the LSI for drive is designed so that the on-resistor of thecommon drive circuit has resistance value of about 500 Ω. Likewise, inthe LCD panel only for displaying, the common electrode is good enoughto be formed of indium tin oxide whose resistance value is about 20-30Ω, while, in the LCD panel with tablet function, indium tin oxide havingresistance value of about 15 Ω/□ is specifically optimum for forming thecommon electrodes.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A display-integrated type tablet device including a display panel which has a display material interposed between segment electrodes and common electrodes crossing each other at right angles and is driven by a duty ratio control type drive method, a detection pen having at a tip of the detection pen an electrode electrostatically coupled with the segment electrodes and the common electrodes of the display panel, a segment electrode drive circuit for driving the segment electrodes, a common electrode drive circuit for driving the common electrodes, a display control circuit for displaying an image on the display panel by controlling the segment electrode drive circuit and the common electrode drive circuit in a period of displaying the image, a detection control circuit for controlling the segment electrode drive circuit to sequentially scan the segment electrodes of the display panel by applying a scanning voltage successively to the segment electrodes and controlling the common electrode drive circuit to sequentially scan the common electrodes by applying a scanning voltage successively to the common electrodes in a coordinate detection period, an x-coordinate detection circuit for detecting an x-coordinate value designated on the display panel by the tip of the detection pen according to an output signal generating timing of the detection pen and a scanning timing of the segment electrodes, and a y-coordinate detection circuit for detecting a y-coordinate value designated on the display panel by the tip of the detection pen according to an output signal generating timing of the detection pen and a scanning timing of the common electrodes,the display-integrated type tablet device comprising a correction voltage generation means for generating a correction voltage for canceling a voltage induced at an electrode closer to the detection pen and placed in an upper position attributed to a scanning voltage applied to an electrode farther to the detection pen and placed in a lower position, the electrodes being one and the other of the segment electrode and the common electrode, thereby canceling the voltage induced at the electrode placed in the upper position by applying the correction voltage generated by the correction voltage generation means to the electrode placed in the upper position at least in a period when the electrode placed in the lower position is scanned.
 2. A display-integrated type tablet device as claimed in claim 1, wherein the correction voltage generation means comprises:a correction signal generation circuit which stores, in an internal memory, digital correction data representing an inverted waveform of a waveform of the signal induced at the upper electrode attributed to the scanning voltage applied to the lower electrode, and reads the correction data to output the correction data as a correction signal; and a correction voltage generation circuit which receives the correction signal from the correction signal generation circuit and generates the correction voltage in an analog form based on the correction signal.
 3. A display-integrated type tablet device as claimed in claim 1, whereineach of the segment electrode drive circuit and the common electrode drive circuit comprises a shift register, and the correction voltage generation means comprises:a correction signal generation circuit which takes in a shift data signal for starting scanning when it is input to an input terminal of a first stage of the shift register of either one of the segment electrode drive circuit and the common electrode drive circuit for scanning the lower electrodes and takes in a pulse signal output from an output terminal of a final stage of the shift register at the time of ending the scanning to generate a binary correction signal having a pulse width substantially equal to the scanning period of the lower electrodes based on the shift data signal and the pulse signal; and a correction voltage generation circuit which receives the correction signal from the correction signal generation circuit to generate the correction voltage in an analog form based on the correction signal.
 4. A display-integrated type tablet device as claimed in claim 1 which comprises an auxiliary electrode facing the lower electrodes, whereinthe correction voltage generation means comprises:a correction signal generation circuit which detects a voltage induced at the auxiliary electrode attributed the scanning voltage applied to the lower electrode and outputs the voltage as a correction signal; and a correction voltage generation circuit which receives the correction signal from the correction signal generation circuit to generate the correction voltage based on an inversion signal of the correction signal.
 5. A display-integrated type tablet device as claimed in claim 1, whereinthe correction voltage generation means comprises:a correction signal generation circuit which detects a current flowing through the upper electrode at the time when the scanning voltage is applied to the lower electrode and outputs the current as a correction signal; and a correction voltage generation circuit which receives the correction signal from the correction signal generation circuit to generate the correction voltage based on the correction signal.
 6. A display-integrated type tablet device as claimed in claim 1, whereinthe correction voltage generation means comprises:a correction signal generation circuit which detects a voltage at the upper electrode at the time when the scanning voltage is applied to the lower electrode and outputs the voltage as a correction signal; and a correction voltage generation circuit which receives the correction signal from the correction signal generation circuit to generate the correction voltage based on the correction signal. 